A major problem in treatment of infections caused by bacteria, particularly hospital acquired infections, is that an increasing number of bacteria are becoming resistant to antibiotics. For example, many strains of Staphylococcus and Enterococcus are now resistant to most of the currently-available antibiotics. Other organisms, such as Pseudomonas, respond poorly. This problem is exacerbated by the ability of many bacteria to transfer resistance to other species of bacteria.
An example of such a bacterium is methicillin resistant Staphylococcus aureus (MRSA) which is a recognised problem in hospitals throughout the world. The mechanism of methicillin resistance is not via β-lactamase production but via altered penicillin binding proteins especially through an alteration in PBP2a. All β-lactams (penicillins, cephalosporins and carbapenems) are therefore ineffective which severely limits the choice of effective antibiotic therapy. Additionally hospital strains of MRSA are usually multi resistant. Often glycopeptides such as vancomycin may be the only effective therapy.
Traditionally the acquisition of such infections was associated with recognised risk factors. These included patients with previous antibiotic use (usually with broad spectrum antimicrobials), previous hospitalisation or exposure to healthcare facilities, those with serious illness, exposure to invasive or surgical procedures or the presence of indwelling catheters and IV drug abuse. Recently, there have been an increasing number of infections caused by MRSA which have been acquired in the community (cMRSA). Most of these patients have none of the classical risk factors associated with such infections. Initially the emergence of cMRSA was in isolated communities, but is now found in nearly all capital cities in Australia (Collignon et al., 1998). It has also been reported in many areas including across the U.S. and Canada, Japan, and New Zealand as well as Australia (Rosenberg, J., 1995; Boyce, J. M., 1998; Herold, B. C. et al., 1998; Lindenmayer, J. M. et al., 1998; Moreno, F. et al., 1995). Community acquired strains of MRSA also differ markedly to hospital acquired strains in their antibiotic sensitivity patterns. In the laboratory testing of resistant bacteria, a requirement for concentrations of antibiotics which are higher than the reported minimum inhibitory concentration (MIC) to inhibit the growth of the organisms is often seen.
One group of antibiotics which are clinically becoming less useful due to acquired resistance are the cephalosporins. Cephalosporins are conventionally believed to act at surface sites on the bacterial cell wall at or near the enzymes responsible for cell wall synthesis. For example in Gram-negative organisms with an outer cell wall, the action of cephalosporins is limited by access to these surface sites in the inner cell wall because of molecular size and other determinants of ability to penetrate porin structures in the outer cell wall, and by the action of enzymes (cephalosporinases) which break down the cephalosporins. These cephalosporinases are largely responsible for the emerging clinical resistance of bacteria to cephalosporins.
Aminoglycoside antibiotics are another class of antibiotics that are affected. While the antibiotics are active against a wide spectrum of organisms, their use has been severely limited by the toxic side effects which occur at the doses required to achieve the desired antibacterial effect.
Thus, there is a need to improve the efficacy of antibiotics, particularly cephalosporins. There is also a need to reduce the toxicity of aminoglycoside antibiotics, particularly gentamicin.
It has conventionally been thought that aminoglycosides exert their antibacterial effects via a strictly intracellular mechanism involving inhibition of ribosomal activity. However, the present inventor has examined data on uptake of radioactively—labelled aminoglycosides, and now proposes that aminoglycosides also act at the cell surface so as to contribute to the process of entry into the cell. Thus the hypothesis underlying the present invention is that an important part of the action(s) of aminoglycoside antimicrobials involve creation of breaches in external cell walls of bacteria and in other external capsular layers or membranes composed of lipopolysaccharide or mucopolysaccharide constituents.
It was thought by the present inventor:                1. that the exposure profiles necessary for this action of aminoglycosides were likely to differ from the concentration-time profiles found to apply to intracellular effect(s), and that novel exposure profiles might be identified which would allow avoidance of toxicity on mammalian systems;        2. that the breaches in external cell walls and capsular membranes and layers of bacteria could facilitate entry and access to sites of action of other antibiotics such as cephalosporins which acted at or near cell surfaces, and additionally, that enzyme degradation of antibiotics (e.g. cephalosporinases) might be by-passed.        
It has now been surprisingly found that the activity of β-lactam antibiotics, including cephalosporins, can be potentiated by the use of a non-toxic amount of an aminoglycoside antibiotic.
While cMRSA strains are resistant to all β-lactams they are still usually sensitive to erythromycin, tetracycline, trimethoprim, ciprofloxacin and gentamicin (Collignon, P. et al., 1998; Daum, R. S. 1998). We have surprisingly found that the combination of gentamicin with a β-lactam provides an effective empiric treatment program for serious infections caused by such organisms while awaiting sensitivity results. This combination also avoids the over use of vancomycin and therefore lowers the risk of developing and spreading vancomycin resistant strains of Staphylococcus aureus that have now been seen in small but increasing numbers in many areas around the world.
The studies detailed herein, using gentamicin, tobramycin, cephazolin and flucloxacillin demonstrate that the concentration-time profiles producing the cell surface effect involve relatively prolonged exposures over many hours, at lower concentrations than those normally used clinically, where rapid onset (bolus) of high concentration exposure has been the characteristic approach to clinical dosing.
The potentiation of cephalosporin action by degrees in excess of 100 fold was also a surprising finding and suggests high efficiency of cell wall porin as exclusion barriers and enzymatic (cephalosporinase) destruction of cephalosporins.